There are various kinds of sensors of a type such that changes in physical parameters which occur in accordance with adsorption of a molecule to be detected (a molecule of an analyte), and these sensors are used in various fields. In order to easily detect the changes in physical parameters, a sensor is generally coated with a layer called as “receptor layer”, and then used for measurements. Since available receptor materials differ depending on physical parameters to be measured, various receptor layers optimized for each sensor have been developed. For example, there is a surface stress sensor that detects a stress that occurs in accordance with adsorption of a molecule of an analyte. For a receptor layer in this kind of sensor, various substances such as self-organized single molecule films, DNAs/RNAs, proteins, antigens/antibodies and polymers are used.
To improve the sensitivity of such sensor, it is effective to optimize the physical and chemical properties of the receptor layer in many cases. For example, regarding a surface stress sensor, as shown in Non-patent Literatures 1 and 2, it is reported that the Young's modulus and film thickness of a receptor material have significant effects. This tendency is represented by the following mathematical formula.
                                          Δ            ⁢                                                  ⁢            z                    =                                                    3                ⁢                                                      l                    c                    2                                    ⁡                                      (                                                                  t                        f                                            +                                              t                        c                                                              )                                                                                                                    (                                          A                      +                      4                                        )                                    ⁢                                      t                    f                    2                                                  +                                                      (                                                                  A                                                  -                          1                                                                    +                      4                                        )                                    ⁢                                      t                    c                    2                                                  +                                  6                  ⁢                                      t                    f                                    ⁢                                      t                    c                                                                        ⁢                          ɛ              f                                      ⁢                                  ⁢                  A          =                                    [                                                E                  f                                ⁢                                  w                  f                                ⁢                                                      t                    f                                    ⁡                                      (                                          1                      -                                              v                        c                                                              )                                                              ]                                      [                                                E                  c                                ⁢                                  w                  c                                ⁢                                                      t                    c                                    ⁡                                      (                                          1                      -                                              v                        f                                                              )                                                              ]                                                          [                  Mathematical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          1                ]            
The above-mentioned formula is for the cantilever-type surface stress sensor shown in Non-patent Literature 1. In the formula, Δz is a deflection of the cantilever, wc is a width of the cantilever, Ic is a length of the cantilever, tc is a thickness of the cantilever, vc is a Poisson's ratio of the cantilever, Ec is a Young's modulus of the cantilever, wf is a width of the receptor layer, tf is a thickness of the receptor layer, vf is a Poisson's ratio of the receptor layer, Ef is a Young's modulus of the receptor layer, and εf is a distortion applied to the receptor layer. When sensitivity (in this case, a deflection amount of the cantilever) is calculated based on this mathematical formula, it is found that the sensitivity significantly depends on the Young's modulus of the receptor layer. Therefore, in order to attain high sensitivity, it is necessary to design a receptor layer having optimal values for physical parameters such as a Young's modulus. Based on the above-mentioned formula, the relationship between the Young's modulus and the deflection amount (sensitivity) is represented by a graph by using the film thicknesses of the receptor layer as parameters, and the graph is shown in FIG. 1. The calculations were conducted under the size of the cantilever of a length of 500 μm, a width of 100 μm and a thickness of 1 μm, and silicon as the material of the cantilever. The following matters are found from this graph.
A. When the film thickness is fixed, there is an optimal value in the Young's modulus of the receptor layer for the sensitivity of the surface stress sensor, and the sensitivity decreases at a Young's modulus that is either higher or lower than the optimal value, and
B. When the thickness of the receptor layer is varied, the optimal Young's modulus changes. Specifically, the optimal Young's modulus is shifted to larger values and the sensitivity is also improved as the thickness of the receptor layer is decreased, whereas, conversely, the sensitivity tends to be not exerted in a region of small Young's moduli.
On the other hand, it is necessary to design a receptor layer specifically having chemical selectivity for this kind of sensor so as to selectively adsorb a molecule of an analyte. Specifically, it is necessary to design a functional group to be contained in the receptor layer and fix the functional group in the receptor layer in a stable state depending on the chemical property of the molecule of the analyte.
As mentioned above, in order to optimize sensitivity and selectivity, which are critically important two elements in a sensor of a type in which a molecule of an analyte is measured, it is generally necessary to optimize physical properties and chemical properties at the same time. However, any effective method by which such optimization is easily attained has not been established, and thus early attainment of such optimization is strongly demanded.